The European Directives on Waste Electrical and Electronic Equipment (WEEE) were implemented in Italy by Legislative Decree 151 of 25 Nov. 2005.
In the specific case of cathode-ray tube television sets, classified as R3, over 75,000 tons of waste material were recovered in the year 2012 by consortiums and specialized companies. The treatments performed on old television sets consist in dismantling them and separating the various types of component materials; the recycling methods are consolidated and efficient for plastic materials, wood-based materials, electronic boards, electrical cables, windings and other metallic materials, while the recycling of the glass parts which constitute the walls of the cathode-ray tube (CRT) is more complex.
A cathode-ray tube consists of the union of two parts: the front part (screen) made of barium/strontium glass and the rear part (named “funnel”) made of lead glass. Previously, these parts were separated, ground and sent to the manufacturing of new similar parts; with the phase-out of the production of cathode-ray tube television sets, it has become necessary to devise disposal treatments of these parts which do not cause environmental problems.
The glass of the screen is currently ground and recycled, for example in the ceramic or glass industries.
Instead, the glass of the funnel is generally coarsely ground, subjected to tumbling to remove the coatings made of harmful toxic materials from the inner walls, and the glass thus treated is then generally sent to the landfill and only rarely recycled as secondary raw material.
In addition to being very costly and representing a possible source of soil pollution in all cases, disposal in authorized landfills constitutes a considerable waste of potentially reusable materials; considering that the funnel weighs about 5 kg in a medium-size CRT and that the minimum lead content in the funnel glass is 17% by weight, from this waste material at least 0.85 kg of lead can be hypothetically recovered per cathode-ray tube. In large-size television sets, the weight of the lead glass can reach even 10 kg, doubling the potential recovery values indicated above.
Some possible methods for the recovery of lead from the funnel of CRTs are described in technical-scientific literature.
For example, the article “A novel process utilizing subcritical water to remove lead from wasted lead silicate glass”, of H. Miyoshi et al., Chemistry Letters, vol. 33 (2004), no. 8, pages 956-957, describes a lead glass attack made using water only at a temperature of 355° C. and a pressure of approximately 240 bar, followed by acid leaching at 100° C. to convert the silicate of the glass into silica and recover a soluble lead salt.
The article “Development of an eco-friendly material recycling process for spent lead glass using a mechanochemical process and Na2EDTA reagent”, R. Sasai et al., Environmental Science Technology, vol. 42, (2008) no. 11, pages 4159-4164, describes a micronization procedure of glass with zirconium oxide balls in presence of a solution of Na2EDTA, with which up to 99% of the lead initially contained in the glass can be extracted.
Finally, the article “Lead extraction from waste funnel cathode-ray tubes glasses by reaction with silicon carbide and titanium nitride”, P. G. Yot et al., Journal of Hazardous Materials, vol. 172 (2009), no. 1, pages 117-123, shows a study on the reduction of Pb(II) to metal lead in the glass; this reduction however is said to occur only partially.
However, the processes described in these articles represent only basic studies and for various reasons are not useful in real practice for the purposes of an industrial disposal process of lead glass to be implemented on large quantities of material.
In the United Kingdom, Nulife Glass Ltd. recovers metal lead from the glass of CRTs using a pyrometallurgical process, claiming a recovery of 95% of the lead present in the glass. However, this process is burdensome from the environmental and energy points of view because it implements the recovery of metallic lead by heating the mass to a temperature higher than 1000° C. and adding carbon as reducing agent; this process inevitably releases lead vapors, which must be abated to prevent environmental contamination. Furthermore, the process residue still contains about 5% of the initial lead, which is still an amount requiring to be disposed of as hazardous waste.
Patent application EP 2455500 A1, assigned to Costech International S.p.A., describes a fully hydrothermal process for the treatment of lead glass, which consists in an alkaline attack of the glass obtaining a suspension containing soluble silicates and insoluble parts, silicate-based in their turn; and subsequent precipitation of the lead from the liquid fraction of this suspension in form of sulfide, PbS, by adding hydrogen sulfide (H2S) or an alkaline sulfide (preferably sodium sulfide, Na2S) to the same. Although effective, this process still display some disadvantages, related in particular to the use of toxic or dangerous materials (such as H2S), and in that the PbS which is recovered must then be treated, in order to obtain metallic lead, using traditional pyrometallurgical processes, which require high temperatures and lead to the formation of sulfur dioxide, SO2, which is a highly polluting substance and which requires suitable recovery/abatement procedures. Another limit of this process is that part of the initial lead, entrapped in the solid fraction of insoluble silicates, is not recovered and remains in the glass.
It is thus still present in the sector the need to avail of a process for the recovery of lead from glass containing it, which is suitable to be adopted on industrial scale and which gives rise to a nearly quantitative recovery of the metal, leaving at most in the silicate byproducts of the process quantities of the metal which are negligible and not harmful.